Technical Field
The present disclosure relates to a manufacturing method of a graphene-based electrochemical sensor and to an electrochemical sensor. In particular, the electrochemical sensor is integrated in a microfluidic system, and is obtained simultaneously with the steps of production of the microfluidic system itself.
Description of the Related Art
Molecule detectors have in the last few years witnessed a considerable development, finding widespread use in a vast range of fields, such as environmental monitoring, food analysis, diagnostics and, more recently, detection of toxic gases and explosive materials.
Notwithstanding the extraordinary potential of use, a considerable limitation of the sensors most commonly used regards the fact that they do not guarantee a sensitivity such as to enable measurement or detection of the presence of single atoms and/or single molecules. One of the main causes that renders problematical achievement of a high resolution is linked to the intrinsic thermal fluctuation in the materials and instruments used during the detection process, which generates an intrinsic noise higher than the useful signal that is to be detected.
In general, a sensor is a device that supplies to the user information on the surrounding environment in which the sensor itself is immersed. It is typically formed by a sensitive element, a transducer, and a data-acquisition system.
There may be distinguished, on the basis of the fields of application, physical sensors, chemical sensors, and biological sensors. A chemical sensor, in particular, is a device able to transform chemical information (such as the concentration of particular elements in the analytes) into a measurable quantity. Following upon the interaction between the analyte (which may be in the gaseous phase or in solution) and the active layer of the sensor, the sensor exerts a receptor and transducer function. The receptor function, which is a consequence of the interaction between the molecules to be detected and the active layer, causes a variation of the chemical and/or physical properties of the material that constitutes the active layer. The transducer function, which is a consequence of the aforementioned variation of the physical/chemical properties, transduces the chemical/physical modification of the active layer into a signal that can be processed, for example an electrical or optical signal.
Preferably, chemical sensors have a number of characteristics that can be summarized in: contained dimensions, presence of a layer able to react in contact with the analyte, sufficiently high speed of response, high capacity of selection of the species, high chemical stability over time and reversibility of the reactions, good mechanical properties of resistance to stresses, and capacity for generating signals of high intensity in the presence of gases or else detectable signals in the presence of small amounts of analytes.
Recently, the development of nanotechnologies applied to sensor systems has opened up new horizons, in particular via the introduction of organic materials deriving from graphite (such as, for example, fullerenes, carbon nanotubes, graphene). Sensor technologies that use thin films, for example made of graphene, have proven particularly effective for this purpose. See, for example, Deepak K. Pandey, Gyan Prakash, and Suprem R. Das, “Graphene Based Sensor Development”—Apr. 28, 2009, which is incorporated herein by reference in its entirety.
The electronic and mechanical properties of graphene are interesting for meeting the previous characteristics and implementing mechanisms of transduction that are particularly effective. The high chemical stability of the 2D lattice, the possibility of functionalizing the surface, the high mobility of the charge carriers (i.e., rapidity of response), the high surface-to-volume ratio, the high conductivity, a reduced defectiveness, and a considerable sensitivity to a wide range of analytes are some of the characteristics that render graphene a material of great interest for providing chemical and physical sensors.
Amongst the types of sensors based upon graphene, an interesting role is played by electrochemical sensors (in particular potentiometric, voltamperometric, conductometric sensors). For example, pH sensors exploit graphene as active channel of a FET, the gate terminal of which is controlled by an electrolytic solution, which plays the role of gate dielectric.
The ions present in the electrolyte cause a transfer of charge at the interface with the graphene that is reflected in a variation of the gate potential, thus modulating the passage of current in the transistor device.
Manufacture of a sensor in which the active layer is made of graphene presents considerably difficult aspects on account of the complexity of the process of synthesis and/or insulation of graphene, up to integration of the graphene layer in the architecture of the sensor.
Such a sensor can be used, for example, for the detection of molecules (analytes) in solution, and to determine the concentration of the molecules in a known volume of fluid. In this case, manufacture of the sensor integrated in a microfluidic system includes production of the sensing device, production of the microfluidic system, and bonding of the parts.
Typically, a microfluidic system is provided through the technique of soft lithography, which enables micro/nanostructured surfaces to be obtained with the use of elastomeric materials. This technique is very widespread and includes the production of a reference mold (master) structured in a way complementary to the fluidic structure that it is desired to obtain (replica) by transfer. The term “soft” regards the use of an elastomer that adapts to the mold replicating the structure thereof. Notwithstanding the great variety of materials available for said applications, the most widely used is PDMS (polydimethylsiloxane) thanks to its particular properties of transparency, biocompatibility, resistance to chemical attacks and to oxidation processes, high dielectric constant, good adhesion on smooth surfaces, high mechanical strength.
Even though it is the technique most widely used, production of the microfluidic system using PDMS typically employs a process flow that is rather long and articulated since it first creates the master (lithographically or electromechanically) and then creates the replica by laying the elastomeric pre-polymer on the master and induces crosslinking thereof by means of thermal treatment that activates the crosslinking agent mixed to the pre-polymer; and finally, separates the replica from the master, taking care not to deform or damage the microfluidic channels. The PDMS mold thus obtained is bonded to a glass made of boron silicate that functions as support and is set on top of the electronic device by means of a technique that includes activation of the surfaces to be bonded by means of an oxygen plasma to favor adhesion thereof, alignment of the two parts, and final pressing. Activation of the surfaces with the oxygen plasma generally renders bonding between the fluidic system and the electronic device irreversible.